The equipment and processes for microgrinding and microdispersion in wet medium are now well known and developed in industries such as:
the mineral industry: fine grinding of particles preground by using traditional processes (in the case of calcium carbonate, titanium oxide, gypsum, kaolin, iron ore and the like) PA1 the industries of paints and inks, dyes, magnetic lacquers and agrochemical compounds, for the dispersion and homogenization of the various liquid and solid constituents. PA1 chemical and colour inertness towards the processed products, PA1 mechanical impact strength PA1 wear resistance PA1 low abrasiveness for the hardware (stirring members, vessels and the like) PA1 high density for a good grinding efficiency PA1 low open porosity for easy cleaning. PA1 round-grained sand PA1 glass beads PA1 metal beads PA1 sintered ceramic beads PA1 fused ceramic beads. PA1 insufficient inertness towards the processed products (contamination of inorganic fillers, greying of paints and the like) PA1 excessively high density, requiring special mills (high energy usage, heating, (mechanical stressing of the hardware). PA1 sintered ceramic beads obtained by a cold forming of ceramic powder and consolidation by firing at high temperature; and PA1 "fused" ceramic beads, that is to say beads obtained by melting ceramic components at very high temperature, forming spherical droplets and solidifying. PA1 40 to 95% of ZrO.sub.2 and HfO.sub.2 ; PA1 at least one of the additional oxides Y.sub.2 O.sub.3 and CeO.sub.2, with the provisos that Y.sub.2 O.sub.3, when present, represents 0.1 to 10% and CeO.sub.2, when present, represents 1 to 15%, the total of Y.sub.2 O.sub.3 and CeO.sub.2 representing 0.1 to 25%; PA1 a quantity of SiO.sub.2 representing 10 to 45% of the composition when CeO.sub.2 is absent from the composition and 0.5 to 45% when CeO.sub.2 is present in the composition. PA1 rare-earth oxides associated in natural ores containing Y.sub.2 O.sub.3 and/or CeO.sub.2 (in the case of use of more economical impure raw materials): from 0 to 10% PA1 Al.sub.2 O.sub.3 in a proportion such that the weight ratio Al.sub.2 O.sub.3 /SiO.sub.2 has a value of 0 to 1.5 PA1 Na.sub.2 O in a proportion such that the weight ratio Na.sub.2 O/SiO.sub.2 has a value of 0 to 0.04 PA1 MgO in a proportion such that the weight ratio MgO/SiO.sub.2 has a value of 0 to 1 PA1 CaO in a proportion such that the weight ratio CaO/SiO.sub.2 has a value of 0 to 1.45. PA1 to decrease the porosity defects (shrinkage holes or bubbles) PA1 to obtain denser beads PA1 to obtain beads with higher impact strength and wear resistance. PA1 the grinding efficiency, using the reduction in diameter of the ground powder PA1 the wear on hardware, using the loss in weight of the stirring discs PA1 bead wear using the loss in weight of the charge of grinding media.
In most cases this equipment and these processes employ dispersing or grinding media of spherical shape and of small diameter (for example beads from 0.3 to 4 mm).
Briefly, these beads must have the following properties:
From a practical view point, a limited number of dispersing or grinding media are found on the market:
Round-grained sand (for example Ottawa sand) is a natural and cheap product. Although used originally, it is employed less and less because it is not suited to modern (pressurized and high-throughput) mills. The sand is actually low in strength, of low density, variable in quality, and is abrasive towards the hardware.
Glass beads have gradually replaced sand as a result of their obvious advantages over the latter: better strength, lower abrasiveness, availability in a wider range of diameters.
Glass beads are widely employed at present, but a demand has appeared for grinding and dispersing media of higher performance: beads of higher impact strength and wear resistance and which are more efficient (denser).
Steel beads, known for a long time, provide a partial answer to this problem, but their use remains marginal for the following reasons:
Ceramic beads are also known. These beads have a better strength than glass beads, a higher density and an excellent chemical inertness. The following may be distinguished:
The great majority of fused beads have a composition of the zirconia-silica (ZrO.sub.2 --SiO.sub.2) type in which the zirconia is crystallized in monoclinic form and the silica (and any additives) forms a glassy phase. These fused ceramic beads have a mixed structure--intimately intermixed crystalline phase and glassy phase--which gives them a low abrasiveness towards the grinding hardware, while permitting optimum grinding properties (mechanical strength, high density, chemical inertness). This glassy phase occupies a volume which is always markedly greater as a percentage than the weight percentage of silica introduced.
A wide range of fused ceramic beads for grinding and dispersion is described in FR-A 2 320 276 or U.S. Pat. No. 4,106,947. This patent covers a wide range of ZrO.sub.2 --SiO.sub.2 compositions and describes the influence of the additional oxides Al.sub.2 O.sub.3, Na.sub.2 O, MgO and CaO.
Although the various fused ceramic beads of the prior art are of good quality, industry always requires products of ever better quality.